1. Field of the Disclosure
The present disclosure relates to an improved method of determining, while drilling in the earth with a drill bit, the velocity of seismic signals and the positions of geologic formations in the earth. More particularly, it relates to a method for improving the quality of the acquired data.
2. Description of the Related Art
Conventional reflection seismology utilizes surface sources and receivers to detect reflections from subsurface impedance contrasts. The obtained image often suffers in spatial accuracy, resolution and coherence due to the long travel paths between source, reflector, and receiver. In particular, due to the two-way passage of seismic signals through a highly absorptive near surface weathered layer with a low, laterally varying velocity, subsurface images are poor quality. To overcome this difficulty, a technique commonly known as vertical seismic profiling (VSP) was developed to image the subsurface in the vicinity of a borehole. With VSP, a surface seismic source is used and signals are received at a single downhole receiver or an array of downhole receivers. This is repeated for different depths of the receiver (or receiver array).
During drilling operations, the drillstring undergoes continuous vibrations. There are prior art methods and devices in which the drill bit itself is used as a seismic source and processing of signals received at a surface location is used for imaging of reflected signals from the earth formation. There are several major drawbacks with this method. The estimation of the source signal is done at the surface by using a sensor at or near the top of the drillstring, near the swivel. The source signal travels a long way through the drillstring enhanced estimation off the source signal may not be accurate. The present disclosure satisfies this need.